Medium conveying apparatus for determining whether a conveyed medium is id card

ABSTRACT

A medium conveying apparatus includes a conveying roller to convey a medium, an imaging device to generate an input image acquired by imaging the conveyed medium, and a processor to extract edge pixels from the input image, detect a degree of roundness at a corner of the medium in the input image based on the edge pixels, determine whether the conveyed medium is an ID card based on the degree of roundness, and control the conveying roller based on a result of the determination or outputs the result of the determination.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 16/985,055, filed Aug. 4, 2020, which claimspriority to and benefit of prior Japanese Patent Application No.2019-167528, filed on Sep. 13, 2019, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

Embodiments discussed in the present specification relate to mediumconveyance.

BACKGROUND

A medium conveying apparatus such as a scanner that conveys a mediumsuch as a document and reads an image of the conveyed medium has afunction of detecting whether or not multi-feed in which a plurality ofmedia are conveyed in an overlapped manner has occurred. In general, themedium conveying app includes an ultrasonic transmitter for transmittingan ultrasonic wave and an ultrasonic receiver for outputting a signalcorresponding to the received ultrasonic wave, and detects multi-feedbased on a signal output by the ultrasonic receiver when the medium isconveyed. However, the reception level of ultrasonic waves whenplastic-made Identification (ID) card is conveyed as a medium is closeto the reception level of ultrasonic waves when multi-feed of sheetsoccurs. Therefore, when the ID card is conveyed, it may be mistakenlydetermined that multi-feed has occurred. Further, in the mediumconveying apparatus, it is also desired to classify an image acquired byreading an ID card and an image acquired by reading another medium.Therefore, in the medium conveying apparatus, it is desired to suitablydetermine whether or not the conveyed medium is an ID card.

In order to acquire an image including the entire document regardless ofa shape of the document, an image forming apparatus that expands areference range representing an outline of the document detected fromthe input image, sets the reference range as the range of the documentimage including the entire document, and outputs the document image, isdisclosed (see Japanese Unexamined Patent Publication (Kokai) No.2018-148362).

SUMMARY

According to some embodiments, a medium conveying apparatus includes aconveying roller to convey a medium, an imaging device to generate aninput image acquired by imaging the conveyed medium, and a processor toextract edge pixels from the input image, detect a degree of roundnessat a corner of the medium in the input image based on the edge pixels,determine whether the conveyed medium is an ID card based on the degreeof roundness, and control the conveying roller based on a result of thedetermination or outputs the result of the determination.

According to some embodiments, a method for determining whether aconveyed medium is an ID card includes conveying a medium by a conveyingroller, generating an input image acquired by imaging the conveyedmedium by an imaging device, extracting edge pixels from the inputimage, detecting a degree of roundness at a corner of the medium in theinput image based on the edge pixels, determining whether the conveyedmedium is an ID card based on the degree of roundness, and controllingthe conveying roller based on a result of the determination oroutputting the result of the determination.

According to some embodiments, a computer program causes a mediumconveying apparatus including a conveying roller to convey a medium, andan imaging device to generate an input image acquired by imaging theconveyed medium, to execute a process including extracting edge pixelsfrom the input image, detecting a degree of roundness at a corner of themedium in the input image based on the edge pixels, determining whetherthe conveyed medium is an ID card based on the degree of roundness, andcontrolling the conveying roller based on a result of the determinationor outputting the result of the determination.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view illustrating a medium conveying apparatus100 according to an embodiment.

FIG. 2 is a diagram for illustrating a conveyance path inside the mediumconveying apparatus 100.

FIG. 3 is a block diagram illustrating a schematic configuration of themedium conveying apparatus 100.

FIG. 4 is a diagram illustrating a schematic configuration of a storagedevice 130 and a processing circuit 140.

FIG. 5 is a flowchart illustrating an operation example of mediumreading processing.

FIG. 6 is a flowchart illustrating an operation example of carddetermination processing.

FIG. 7A is a schematic diagram for illustrating a distance between eachintersection and an edge pixel.

FIG. 7B is a schematic diagram for illustrating a sheet P on whichcorner folding has occurred.

FIG. 7C is a schematic diagram for illustrating a sheet P on whichcorner folding has occurred.

FIG. 8 is a flowchart illustrating the operation example of themulti-feed determination processing.

FIG. 9 is a schematic diagram for illustrating a characteristic of anultrasonic signal.

FIG. 10 is a flowchart illustrating another operation example of carddetermination processing.

FIG. 11A is a schematic diagram for illustrating a second orderderivative value of each edge pixel.

FIG. 11B is a schematic diagram for illustrating a second orderderivative value of each edge pixel.

FIG. 11C is a schematic diagram for illustrating a second orderderivative value of each edge pixel.

FIG. 11D is a schematic diagram for illustrating a second orderderivative value of each edge pixel.

FIG. 12 is a flowchart illustrating still another operation example ofcard determination processing.

FIG. 13A is a schematic diagram for illustrating a second derivativevalue of each medium width.

FIG. 13B is a schematic diagram for illustrating a second derivativevalue of each medium width.

FIG. 14 is a flowchart illustrating another operation example of themulti-feed determination processing.

FIG. 15 is a diagram illustrating a schematic configuration of aprocessing circuit 240 in another medium conveying apparatus.

DESCRIPTION OF EMBODIMENTS

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory, andare not restrictive of the invention, as claimed.

Hereinafter, a medium conveying apparatus, a method and acomputer-readable, non-transitory medium storing a computer programaccording to an embodiment, will be described with reference to thedrawings. However, it should be noted that the technical scope of theinvention is not limited to these embodiments, and extends to theinventions described in the claims and their equivalents.

FIG. 1 is a perspective view illustrating a medium conveying apparatus100 configured as an image scanner. The medium conveying apparatus 100conveys and images a medium being a document. A medium is paper, thickpaper, a card, a brochure, a passport, etc. For example, a card is aplastic resin card (plastic card). The card is, for example, anidentification card having IC (Integrated Circuit) chips or magneticstripes in which authentication information is stored, or having auser's name, photograph, etc., printed on its surface. Particularly, acard is an identification (ID) card defined by InternationalOrganization for Standardization (ISO)/International ElectrotechnicalCommission (IEC) 7810. A card may be another type of card. The mediumconveying apparatus 100 may be a fax machine, a copying machine, amultifunctional peripheral (MFP), etc.

The medium conveying apparatus 100 includes a lower housing 101, anupper housing 102, a medium tray 103, an ejection tray 104, an operationdevice 105, and a display device 106.

The upper housing 102 is located in a position covering a top surface ofthe medium conveying apparatus 100, and is engaged with the lowerhousing 101. The medium tray 103 is engaged with the lower housing 101in such a way as to be able to place a medium to be conveyed. Theejection tray 104 is engaged with the lower housing 101 in such a way asto be able to hold an ejected medium.

The operation device 105 includes an input device such as a button, andan interface circuit acquiring a signal from the input device, receivesan input operation by a user, and outputs an operation signal based onthe input operation by the user. The display device 106 includes adisplay including a liquid crystal or organic electro-luminescence (EL),and an interface circuit for outputting image data to the display, anddisplays the image data on the display.

FIG. 2 is a diagram for illustrating a conveyance path inside the mediumconveying apparatus 100.

The conveyance path inside the medium conveying apparatus 100 includes afirst medium sensor 111, a feed roller 112, a brake roller 113, anultrasonic transmitter 114 a, an ultrasonic receiver 114 b, a firstconveyance roller 115, a second conveyance roller 116, a first mediumsensor 117, a first imaging device 118 a, a second imaging device 118 b,a third conveyance roller 119, and a fourth conveyance roller 120, etc.The numbers of each roller is not limited to one, and may be plural.

A top surface of the lower housing 101 forms a lower guide 107 a of aconveyance path of a medium, and a bottom surface of the upper housing102 forms an upper guide 107 b of the conveyance path of a medium. Anarrow A1 in FIG. 2 indicates a medium conveying direction. An upstreamhereinafter refers to an upstream in the medium conveying direction A1,and a downstream refers to a downstream in the medium conveyingdirection A1.

The first medium sensor 111 includes a contact detection sensor anddetects whether or not a medium is placed on the medium tray 103. Thefirst medium sensor 111 generates and outputs a medium detection signalchanging the signal value between a state in which a medium is placed onthe medium tray 103 and a state in which a medium is not placed.

The ultrasonic transmitter 114 a and the ultrasonic receiver 114 b areexamples of an ultrasonic transmission module and an ultrasonicreception module, respectively. The ultrasonic transmitter 114 a and theultrasonic receiver 114 b are provided on the downstream side of thefeed roller 112 and the brake roller 113, and also on the upstream sideof the first conveyance roller 115 and the second conveyance roller 116,i.e., on the upstream side of the first imaging device 118 a and thesecond imaging device 118 b. The ultrasonic transmitter 114 a and theultrasonic receiver 114 b are located close to the conveyance path of amedium in such a way as to face one another with the conveyance path inbetween. The ultrasonic transmitter 114 a outputs an ultrasonic wave. Onthe other hand, the ultrasonic receiver 114 b receives an ultrasonicwave being transmitted by the ultrasonic transmitter 114 a and passingthrough a medium, and generates and outputs an ultrasonic signal beingan electric signal corresponding to the received ultrasonic wave. Theultrasonic transmitter 114 a and the ultrasonic receiver 114 b may behereinafter collectively referred to as an ultrasonic sensor 114.

The second medium sensor 117 is arranged on the downstream side of thefirst conveyance roller 115 and the second conveyance roller 116, and onthe upstream side of the first imaging device 118 a and the secondimaging device 118 b, in the medium conveying direction A1. The secondmedium sensor 117 includes a light emitter and a light receiver that areprovided on one side of the medium conveyance path (upper housing 102).The second medium sensor 117 includes a reflection member, such as amirror, provided at a position facing the light emitter and the lightreceiver with the conveyance path in between. The light emitter projectslight toward the conveyance path. On the other hand, the light receiverreceives light projected by the light emitter and reflected by thereflection member, and generates and outputs a second medium signalbeing an electric signal based on intensity of the received light. Whena medium exists at a position of the second medium sensor 117, lightprojected by the light emitter is shaded by the medium. A signal valueof the second medium signal varies between a state in which a mediumexists at the position of the second medium sensor 117 and a state inwhich a medium does not exist. Thereby, the second medium sensor 117detects whether or not the medium exists at the position of the secondmedium sensor 117, and detects the conveyed medium. The light emitterand the light receiver of the second medium sensor 117 may be providedat positions facing one another with the conveyance path in between, andthe reflection member may be omitted.

The first imaging device 118 a includes a line sensor based on aunity-magnification optical system type contact image sensor (CIS)including an imaging element based on a complementary metal oxidesemiconductor (CMOS) linearly located in a main scanning direction.Further, the first imaging device 118 a includes a lens for forming animage on the imaging element, and an A/D converter for amplifying andanalog-digital (ND) converting an electric signal output from theimaging element. The first imaging device 118 a images a front surfaceof a medium conveyed by the feed roller 112, the brake roller 113, thefirst conveyance roller 115, and the second conveyance roller 116. Thefirst imaging device 118 a sequentially generates and outputs a lineimage acquired by imaging an area of a conveyed medium facing the linesensor, at certain intervals. Specifically, a pixel count of a lineimage in a vertical direction (subscanning direction) is 1, and a pixelcount in a horizontal direction (main scanning direction) is larger than1.

Similarly, the second imaging device 118 b includes a line sensor basedon a unity-magnification optical system type CIS including an imagingelement based on a CMOS linearly located in a main scanning direction.Further, the second imaging device 118 b includes a lens for forming animage on the imaging element, and an ND converter for amplifying and NDconverting an electric signal output from the imaging element. Thesecond imaging device 118 b images the back side of a medium beingconveyed by the feed roller 112, the brake roller 113, the firstconveyance roller 115, and the second conveyance roller 116. The secondimaging device 118 b sequentially generates and outputs a line imageacquired by imaging an area of a conveyed medium facing the line sensor,at certain intervals.

Only either of the first imaging device 118 a and the second imagingdevice 118 b may be located in the medium conveying apparatus 100 andonly one surface of a medium may be read. Further, a line sensor basedon a unity-magnification optical system type CIS including an imagingelement based on charge coupled devices (CCDs) may be used in place ofthe line sensor based on a unity-magnification optical system type CISincluding an imaging element based on a CMOS. Further, a line sensorbased on a reduction optical system type line sensor including animaging element based on CMOS or CCDs. The first imaging device 118 aand the second imaging device 118 b may be collectively referred to asimaging devices 118. The imaging device 118 is an example of imagingmodule. The one or a plurality of line images generated by the imagingdevice 118 are an example of an input image acquired by imaging aconveyed medium.

A medium placed on the medium tray 103 is conveyed between the lowerguide 107 a and the upper guide 107 b in the medium conveying directionA1 by the feed roller 112 rotating in a direction of an arrow A2 in FIG.2. When a medium is conveyed, the brake roller 113 rotate in a directionof an arrow A3. By the workings of the feed rollers 112 and the brakeroller 113, when a plurality of media are placed on the medium tray 103,only a medium in contact with the feed roller 112, out of the mediaplaced on the medium tray 103, is separated. Consequently, the mediumconveying apparatus 100 operates in such a way that conveyance of amedium other than the separated medium is restricted (prevention ofmulti-feed).

A medium is fed between the first conveyance roller 115 and the secondconveyance roller 116 while being guided by the lower guide 107 a andthe upper guide 107 b. The medium is fed between the first imagingdevice 118 a and the second imaging device 118 b by the first conveyanceroller 115 and the second conveyance roller 116 rotating in directionsof an arrow A4 and an arrow A5, respectively. The medium read by theimaging devices 117 is ejected on the ejection tray 104 by the thirdconveyance roller 119 and the fourth conveyance roller 120 rotating indirections of an arrow A6 and an arrow A7, respectively. The feed roller112, the brake roller 113, the first conveyance roller 115, and thesecond conveyance roller 116 are examples of a conveying roller forconveying a medium.

FIG. 3 is a block diagram illustrating a schematic configuration of themedium conveying apparatus 100.

The medium conveying apparatus 100 further includes a driving device121, an interface device 122, a storage device 130, and a processingcircuit 140, etc., in addition to the configuration described above.

The driving device 121 includes one or a plurality of motors, andconveys a medium by rotating the feed roller 112, the brake roller 113,and the first to fourth conveyance rollers 115, 116, 119, and 120, by acontrol signal from the processing circuit 140.

For example, the interface device 122 includes an interface circuitconforming to a serial bus such as universal serial bus (USB), iselectrically connected to an unillustrated information processing device(for example, a personal computer or a mobile information terminal), andtransmits and receives an input image and various types of information.Further, a communication module including an antenna transmitting andreceiving wireless signals, and a wireless communication interfacedevice for transmitting and receiving signals through a wirelesscommunication line in conformance with a predetermined communicationprotocol may be used in place of the interface device 122. For example,the predetermined communication protocol is a wireless local areanetwork (LAN).

The storage device 130 includes a memory device such as a random accessmemory (RAM) or a read only memory (ROM), a fixed disk device such as ahard disk, or a portable storage device such as a flexible disk or anoptical disk. Further, the storage device 130 stores a computer program,a database, a table, etc., used for various types of processing in themedium conveying apparatus 100. The computer program may be installed onthe storage device 130 from a computer-readable, non-transitory mediumsuch as a compact disc read only memory (CD-ROM), a digital versatiledisc read only memory (DVD-ROM), etc., by using a well-known setupprogram, etc.

For example, the processing circuit 140 is a processor, such as acentral processing unit (CPU). The processing circuit 140 operates inaccordance with a program previously stored in the storage device 130.The processing circuit 140 may be a digital signal processor (DSP), alarge scale integration (LSI), an application specific integratedcircuit (ASIC), a field-programmable gate array (FPGA), etc.

The processing circuit 140 is connected to the operation device 105, thedisplay device 106, the first medium sensor 111, the ultrasonic sensor114, the second medium sensor 117, the imaging devices 118, the drivingdevice 121, the interface device 122, the storage device 130, etc., andcontrols each of these units. The processing circuit 140 performs drivecontrol of the driving device 121, imaging control of the imagingdevices 117, etc., acquires an image, and transmits the image to anunillustrated information processing apparatus through the interfacedevice 142. Further, the processing circuit 140 determines whether ornot a conveyed medium is an ID card based on the input image generatedby the imaging device 118, controls the conveying roller based on aresult of the determination, or transmits the result of thedetermination to the information processing apparatus via the interfacedevice 122.

FIG. 4 is a diagram illustrating schematic configurations of the storagedevice 130 and the processing circuit 140.

As illustrated in FIG. 4, the storage device 130 stores a controlprogram 131, an extraction program 132, a detection program 133, and adetermination program 134, etc. Each of these programs is a functionalmodule implemented by software operating on a processor. The processingcircuit 140 reads each program stored in the storage device 130 andoperates in accordance with each read program. Consequently, theprocessing circuit 140 functions as a control module 141, an extractionmodule 142, a detection module 143, and a determination module 144.

FIG. 5 is a flowchart illustrating an operation example of mediumreading processing in the medium conveying apparatus 100.

Referring to the flowchart illustrated in FIG. 5, an operation exampleof the medium reading processing in the medium conveying apparatus 100will be described below. The operation flow described below is executedmainly by the processing circuit 140 in cooperation with each element inthe medium conveying apparatus 100, in accordance with a programpreviously stored in the storage device 130. The operation flowillustrated in FIG. 5 is periodically executed.

First, the control module 141 stands by until an instruction to read amedium is input by a user by use of the operation device 105, and anoperation signal instructing to read the medium is received from theoperation device 105 (step S101).

Next, the control module 141 determines whether or not a medium isplaced on the medium tray 103, based on a first medium signal receivedfrom the first medium sensor 111 (step S102).

When a medium is not placed on the medium tray 103, the control module141 returns the processing to step S101 and stands by until newlyreceiving an operation signal from the operation device 105.

On the other hand, when a medium is placed on the medium tray 103, thecontrol module 161 drives the driving device 121, rotates the feedrollers 112, the brake rollers 113, and the first to fourth conveyancerollers 115, 116, 119, and 120, and feeds and conveys the medium (stepS103).

Next, the control module 141 causes the imaging device 118 to startimaging the medium, and determines whether or not a predetermined amountof the medium has been imaged by the imaging device 118 (step S104). Thepredetermined amount is set to an amount sufficient to determine theroundness of the corner of the ID card. For example, the predeterminedamount is set to an amount corresponding to the entire conveyed medium.The predetermined amount may be an amount that the entire roundness ofthe corners of at least the front edge of the ID card (end portion ofthe downstream side) is imaged.

The control module 141 acquires a second medium signal from the secondmedium sensor 117 and determines whether or not a medium exists at theposition of the second medium sensor 117 based on the acquired secondmedium signal. When a signal value of the second medium signal changesfrom a value indicating nonexistence of a medium to a value indicatingexistence of a medium, the control module 141 determines that the frontedge of the medium passes the position of the second medium sensor 117.Then, when a signal value of the second medium signal changes from thevalue indicating existence of a medium to the value indicatingnonexistence of a medium, the control module 141 determines that therear edge of the medium passes the position of the second medium sensor117. The control module 141 determines that a predetermined amount ofthe medium has been imaged by the imaging device 118 when apredetermined period elapses after determining that the rear edge of themedium passes the position of the second medium sensor 117. The controlmodule 141 may determine that a predetermined amount of the medium hasbeen imaged by the imaging device 118 when the conveyance periodcorresponding to the predetermined amount has elapsed after determiningthat the rear edge of the medium passes the position of the secondmedium sensor 117.

Further, the control module 141 may determine whether or not apredetermined amount of the medium has been imaged by acquiring lineimages sequentially generated by the imaging device 118 and analyzingthe acquired line images. In this case, the control module 141determines whether or not the medium is included in the line image byusing a known image processing technique, and when the control module141 determines that the entire medium or a predetermined amount of themedium is included in the line image, the control module 141 determinesthat the predetermined amount of the medium has been imaged by theimaging device 118.

The control module 141 waits until a predetermined amount of the mediumis imaged by the imaging device 118. When the imaging device 118 hasimaged a predetermined amount of the medium, the processing circuit 140executes a card determination processing (step S105). In the carddetermination processing, the processing circuit 140 determines whetheror not the conveyed medium is an ID card. Details of the carddetermination processing will be described later.

Next, the control module 141 determines whether or not it has beendetermined in a multi-feed determination process (described later) thatmulti-feed of the medium has occurred (step S106).

When it is determined that multi-feed has occurred, the control module141, as an abnormality process, stops the driving device 121, stopsconveying the medium, and notifies the user that an abnormality hasoccurred, by using a speaker (not shown), LEDs, etc., (step S107). Then,the control module 141 ends the series of steps. The control module 141may stop conveying the next medium after eject of the currently conveyedmedium, instead of immediately stopping conveying the medium, as theabnormal processing.

On the other hand, when it is not determined that multi-feed hasoccurred, the control module 141 determines whether or not the entireconveyed medium has been imaged (step S108). For example, based on thesecond medium signal acquired from the second medium sensor 117, thecontrol module 141 determines that the entire medium has been imagedwhen a predetermined period of time has elapsed after determining thatthe rear end of the medium has passed through the position of the secondmedium sensor 117. The control module 141 may determine that the entireconveyed medium has been imaged, when the control module 141 acquires apredetermined number of line images from the imaging device 118.

When the entire conveyed medium has not been imaged, the control module141 returns the processing to step S106 and repeats the processing instep S106 to S108.

On the other hand, when the entire conveyed medium has been imaged, thecontrol module 141 acquires a read image by combining all the acquiredline images (step S109).

Next, the control module 141 outputs the read image and the result ofthe determination in the card determination process, i.e., theinformation indicating whether or not the medium included in the readimage is the ID card, by transmitting the read image to an informationprocessing apparatus (not shown) via the interface apparatus 122 (stepS110). When receiving the read image and the result of thedetermination, the information processing apparatus classifies whetherthe read image corresponds to the ID card or another medium according tothe received result of the determination, and stores each read image ina storage area such as a corresponding folder.

Next, the control module 141 determines whether or not a medium remainson the medium tray 103 based on a first medium signal acquired from thefirst medium sensor 111 (step S111). When a medium remains on the mediumtray 103, the control module 171 returns the processing to step S104 andrepeats the processing in steps S104 to S107.

On the other hand, when a medium does not remain on the medium tray 103,the control module 141 stops the driving device 121 (step S112) and endsthe series of steps.

FIG. 6 is a flowchart illustrating an operation example of the carddetermination processing.

The flow of operations shown in FIG. 6 is performed in step S105 of theflow chart shown in FIG. 5.

First, the control module 141 acquires the line image generated by theimaging device 118 up to the present time as the input image (stepS201).

Next, the extraction module 142 extracts edge pixels from the inputimage (step S202). For each horizontal line (main scanning directionline) in the input image, the extraction module 142 calculates anabsolute value (hereinafter referred to as an adjacent difference value)of a difference in gradation values between each pixel and a pixeladjacent to the right side of each pixel or a pixel separated by apredetermined distance from each pixel to the right side, in order fromthe left end of the input image. The gradation value is a luminancevalue or a color value. The extraction module 142 extracts the first(leftmost) pixel whose adjacent difference value is equal to or largerthan the edge threshold value as the left edge pixel. The edge thresholdvalue can be set, for example, to a difference (e.g., 20) of gradationvalues that a person can visually distinguish a difference in brightnesson an image.

Similarly, for each horizontal line in the input image, the extractionmodule 142 calculates an adjacent difference value between each pixeland a pixel on the left side of each pixel, in order from the right endof the input image, and extracts the first pixel whose adjacentdifference value is equal to or larger than the edge threshold value asthe right edge pixel. For each horizontal line in the input image, theextraction module 142 may calculate an adjacent difference value betweeneach pixel and a pixel on the right side of each pixel, in order fromthe left end of the input image, and extract the last pixel whoseadjacent difference value is equal to or larger than the edge thresholdvalue as the right edge pixel. Further, for each vertical line in theinput image, the extraction module 142 calculates an adjacent differencevalue between each pixel and a pixel on the lower side of each pixel, inorder from the upper end of the input image, and extracts the firstpixel whose adjacent difference value is equal to or larger than theedge threshold value as the upper edge pixel. Further, for each verticalline in the input image, the extraction module 142 calculates anadjacent difference value between each pixel and a pixel on the upperside of each pixel, in order from the lower end of the input image, andextracts the first pixel whose adjacent difference value is equal to orlarger than the edge threshold value as the lower edge pixel. For eachvertical line in the input image, the extraction module 142 maycalculate an adjacent difference value between each pixel and a pixel onthe lower side of each pixel, in order from the upper end of the inputimage, and extract the last pixel whose adjacent difference value isequal to or larger than the edge threshold value as the lower edgepixel.

The extraction module 142 may calculate the absolute value of thedifference between the gradation values of the two pixels separated by apredetermined distance or both adjacent pixels in the horizontal orvertical direction of each pixel as the adjacent difference value.Further, when the gradation value of a specific pixel is less than thethreshold value and the gradation value of a pixel adjacent to thespecific pixel or a pixel separated by a predetermined distance is equalto or larger than the threshold value, the extraction module 142 mayextract the specific pixel as an edge pixel.

Next, the detection module 143 detects two straight lines substantiallyperpendicular to each other from the edge pixels extracted by theextraction module 142 (step S203). First the detection module 143detects a straight line corresponding to the left side of the mediumfrom the left edge pixel, detects a straight line corresponding to theright side of the medium from the right edge pixel, detects a straightline corresponding to the upper side of the medium from the upper edgepixel, detects a straight line corresponding to the lower side of themedium from the lower edge pixel. The detection module 143 detects astraight line using the least squares method. The detection module 143may detect the straight line by using the Hough transform.

Further, the detection module 143 may detect a straight line, withoutusing a specific left edge pixel and a specific right edge pixels amongthe left edge pixels and the right edge pixels, whose horizontalpositions are separated from the horizontal position of a left edgepixel and a right edge pixel adjacent in the vertical direction by apredetermined distance or more from the specific left edge pixel and thespecific right edge pixel. Similarly, the detection module 143 maydetect a straight line without using a specific upper edge pixel and aspecific lower edge pixels among the upper edge pixels and the loweredge pixels whose vertical positions are separated from the verticalpositions of a upper edge pixels and a lower edge pixel adjacent in thehorizontal direction by a predetermined distance or more from thespecific upper edge pixel and the specific lower edge pixel. Thus, thedetection module 143 can detect a straight line so as not to be affectedby the upper and lower edges of the medium detected as the left edgepixel and the right edge pixel or the left and right edges of the mediumdetected as the upper edge pixel and the lower edge pixel when themedium is conveyed inclined.

Next, the detection module 143 extracts two straight lines substantiallyorthogonal to each other from the detected plurality of straight lines.First, the detection module 143 selects one straight line from thestraight lines corresponding to the left side of the medium, andextracts a straight line substantially parallel (for example, within±3°) to the selected straight line from the straight lines correspondingto the right side of the medium. Next, the detection module 143 extractsa straight line substantially orthogonal (for example, within ±3° withrespect to 90°) to the straight line selected from the straight linescorresponding to the left side of the medium, from the straight linescorresponding to the upper side and the lower side of the medium,respectively. The detection module 143 extracts straight lines of acombination in which the area of the region surrounded by each straightline is the largest among the combinations of straight lines satisfyingthe above conditions. The detection module 143 detects straight linescorresponding to each of the extracted left and upper sides, straightlines corresponding to each of the upper and right sides, straight linescorresponding to each of the right and lower sides, and straight linescorresponding to each of the lower and left sides, respectively, as twostraight lines substantially perpendicular to each other.

Next, the detection module 143 detects the intersection of detected twostraight lines substantially perpendicular to each other (step S204).

Next, the detection module 143, for detected intersection, calculatesthe distances between each intersection and one or more predeterminededge pixel (step S205). The predetermined edge pixels are edge pixelslocated inside each intersection, i.e., on the center side of the areasurrounded by the detected straight line from the detected intersection.Alternatively, the predetermined edge pixels are edge pixels locatedwithin a predetermined range from each intersection. Alternatively, thepredetermined edge pixels are edge pixels from the first predeterminedorder to the second predetermined order in the order closest to eachintersection among all edge pixels. The first predetermined order is setto the first or subsequent order. By setting the first predeterminedorder to the second or subsequent ranks, it is suppressed that the edgepixels detected by noise, etc., are extracted as the predetermined edgepixels. The second predetermined order is the order of the firstpredetermined order or subsequent order. That is, the detection module143, for detected intersection, calculates the distances between eachintersection and one or more predetermined edge pixels.

The detection module 143, as a distance between each intersection andeach predetermined edge pixel, calculates a Euclidean distance betweeneach intersection and each predetermined edge pixel. The detectionmodule 143, as the distance between each intersection and eachpredetermined edge pixel, may calculate a Manhattan distance betweeneach intersection and each predetermined edge pixel, i.e. the sum of thedistance in the horizontal direction and the distance in the verticaldirection. Thus, the detection module 143 can efficiently and accuratelycalculate the distance between each intersection and each predeterminededge pixel. Further, the detection module 143, as the distance betweeneach intersection and each predetermined edge pixel, may calculate aChebyshev distance between each intersection and each predetermined edgepixel, i.e., the larger distance among the distance in the horizontaldirection and the distance in the vertical direction. Thus, thedetection module 143 can efficiently calculate the distance between eachintersection and each predetermined edge pixel.

FIG. 7A is a schematic diagram for illustrating the distance betweeneach intersections and each predetermined edge pixel.

The FIG. 7A shows an input image 700 in which the ID card C is imaged.In the example shown in FIG. 7A, the edge pixel E is extracted from theouter peripheral portion of the ID-card C, a straight line L1corresponding to the left side, a straight line L2 corresponding to theupper side, a straight line L3 corresponding to the right side and astraight line L4 corresponding to the lower side is detected from eachedge pixel E. Further, the first edge pixels E1, the second edge pixelsE2, and the third edge pixels E3 in the order closer from theintersection point V of the straight line L1 and the straight line L2are extracted as predetermined edge pixels, and each distance D1 to D3from the intersection point V to each edge pixel El to E3 is calculated.

As shown in FIG. 7A, since each corner of the ID card has a certaindegree of roundness, distances between each intersection point V andpredetermined edge pixels E1 to E3 in the input image 700 acquired byimaging the ID card have a certain degree of size. On the other hand,since each angle of the ordinary paper such as PPC paper has a rightangle, the distance between each intersection point and a predeterminededge pixel is close to 0 in the input image acquired by imaging theordinary paper. Therefore, the detection module 143 can accuratelydetect the degree of roundness at the corner of the medium included inthe input image based on the distance between each intersection and eachpredetermined edge pixel.

Next, the detection module 143 detects, for detected intersection point,the degree of roundness at the corner of the medium in the input imagebased on the distance between each intersection and each predeterminededge pixel (step S206). For example, the detection module 143calculates, for detected intersection, a statistical value (averagevalue, median value, mode value, maximum value or minimum value, etc.)of each distance between each intersection and each predetermined edgepixel. The degree of roundness is a degree close to the ideal value ofroundness at the corner of the ID card, and the detection module 143calculates the degree of roundness so that the calculated statisticalvalue becomes higher as it is close to the ideal value of the ID card.For example, the detection module 143 calculates the inverse of thedifference between the calculated statistical value and the ideal value,or a value acquired by subtracting the difference from the upper limitvalue, as the degree of roundness.

The ideal value of the roundness at the corner of the ID card is setbased on, for example, the size of the ID card defined by ISO/IEC7810.In ISO/IEC7810 or its additional provisions (ISO/IEC7813, etc.), theradius (R) of roundness of the corner of the ID-card is specified at3.18 mm. Thus, when the conveyed medium is an ID card, the idealdistance between each intersection and each predetermined edge pixel isset to (2^(1/2)−1)×3.18 [mm]. The detection module 143 may set thedegree of roundness to 1 when the difference between the statisticalvalue and the ideal value is equal to or less than the predeterminedthreshold value, and may set the degree of roundness to 0 when thedifference between the statistical value and the ideal value is largerthan the predetermined threshold value. The detection module 143 may setthe degree of roundness to 1 when the statistical value is equal to ormore than the ideal value, and may set the degree of roundness to 0 whenthe statistical value is less than the ideal value.

The detection module 143 may calculate the degree of roundness based onthe degree of change of the Manhattan distance between each intersectionand a plurality of edge pixels located within a predetermined range fromeach intersection.

FIG. 7B and FIG. 7C are schematic diagrams for illustrating a sheet P inwhich corner folding (corner bending) has occurred.

FIG. 7B shows a part of an input image acquired by imaging an inputimage of a sheet P on which corner bending has occurred. As shown inFIG. 7B, since the distance D′ between the intersection point V′detected from the outer shape of the paper P and the predetermined edgepixel E′ has a certain degree of magnitude in the paper P in whichcorner bending has occurred, there is a possibility that the distance D′becomes close to an ideal value. However, since the corners of the paperare usually bent in a straight line, the Manhattan distance (the sum ofthe distance in the horizontal direction and the distance in thevertical direction) between the intersection and a plurality ofpredetermined edge pixels changes linearly. Further, when the shape ofthe folded portion is isosceles triangle, the Manhattan distance betweenthe intersection and a plurality of predetermined edge pixels does notchange.

FIG. 7C shows graph 720 showing the change of Manhattan distance betweenthe intersection and each edge pixel. The horizontal axis of graph 720shows the distance in the horizontal direction from the intersection,and the vertical axis of graph 720 shows the Manhattan distance from theintersection. Graph 721 shows the distribution of the Manhattandistances for each of the edge pixels located within a predeterminedrange from the intersection point V in the ID card C shown in FIG. 7A.Graph 722 shows the distribution of the Manhattan distances for each ofthe edge pixels located within a predetermined range from theintersection point V′ in the paper P shown in FIG. 7B. As shown in FIG.7C, in an ID card C with some roundness in the corners, the Manhattandistance between the intersection and each edge pixel varies to draw agentle curve with respect to the change of the distance in thehorizontal direction. On the other hand, in the paper P in which thecorner folding occurs, the Manhattan distance between the intersectionpoint and each edge pixel varies linearly with respect to the change ofthe distance in the horizontal direction.

First, the detection module 143 extracts a pair of two edge pixelsadjacent to each other in a predetermined direction (horizontal orvertical direction) among a plurality of edge pixels located within apredetermined range from the intersection. The detection module 143calculates, for extracted pair, the first degree of change of thedistance which is a degree of change of the Manhattan distance betweenthe intersection point and each edge pixel. The detection module 143, asthe first degree of change of the distance, calculates the absolutevalue of the ratio (slope) of the change amount of the Manhattandistance to the change amount of the distance in the horizontaldirection between the intersection and each edge pixel.

Next, the detection module 143 extracts two sets of pairs of theextracted two edge pixels adjacent to each other in a predetermineddirection among the pairs of the extracted two edge pixels. Thedetection module 143, for extracted pair, calculates a second degree ofchange of the distance which is a degree of change of the first degreeof change of the distance. The detection module 143, as the seconddegree of change of the distance, calculates the absolute value of theratio of the change amount of the first degree of change of thedistance, with respect to the change amount of the distance in thehorizontal direction between the intersection and each edge pixel (usedwhen calculating the first degree of change of the distance). The seconddegree of change of the distance calculated in this way corresponds to avalue (second order derivative value) acquired by second orderderivative of the Manhattan distance between each intersection and eachedge pixel with respect to the distance in the horizontal directionbetween each intersection and each edge pixel. The detection module 143may calculate the absolute value of the change amount of the firstdegree of change of the distance as the second degree of change of thedistance.

The detection module 143 calculates a statistical value (average value,median value, mode value, maximum value or minimum value, etc.) of thesecond order derivative value calculated for each pair. The detectionmodule 143 corrects the degree of roundness so that the degree ofroundness becomes larger as the calculated statistical value becomeslarger, and the degree of roundness becomes smaller as the calculatedstatistical value becomes smaller. The detection module 143 may use thecalculated statistical value as it is as the degree of roundness.

By calculating the degree of roundness based on the degree of change ofthe Manhattan distance between each intersection and a plurality of edgepixels located within a predetermined range from each intersection, themedium conveying device 100 can suppress the sheet on which the cornerfolding has occurred from being erroneously determined as an ID card.

As described above, the detection module 143 detects the degree ofroundness at the corner of the medium in the input image based on theedge pixels extracted by the extraction module 142.

Next, the determination module 144 determines whether or not the degreeof roundness detected by the detection module 143 is equal to or lessthan the roundness threshold value (step S207). The roundness thresholdvalue is set to a value between the degree of roundness calculated forthe ID card and the degree of roundness calculated for the paper by aprior experiment.

When the degree of roundness is larger than the roundness thresholdvalue, the determination module 144 determines that the conveyed mediumis an ID card (step S208), and ends a series of steps. On the otherhand, when the degree of roundness is equal to or less than theroundness threshold value, the determination module 144 determines thatthe conveyed medium is not an ID card (step S209), and ends a series ofsteps. The determination module 144 determines that the medium is an IDcard when all degree of roundness detected for each intersection(corner) is larger than the roundness threshold value, and determinesthat the medium is not an ID card when at least one degree of roundnessdetected for each intersection is equal to or less than the roundnessthreshold value. The determination module 144 may determine that themedium is an ID card when the degree of roundness of a predeterminednumber (e.g., 1) or more is larger than the roundness threshold value,otherwise the determination module 144 may determine that the medium isnot an ID card. For example, by setting the predetermined number to twoor more, the determination module 144 can suppress erroneousdetermination of a sheet in which a corner bend occurs in only onecorner as an ID card. In this manner, the determination module 144determines whether or not the conveyed medium is an ID card based on thedegree of roundness.

Instead of calculating the degree of roundness for all the corners ofthe medium, the detection module 143 may calculate the degree ofroundness for at least one corner. In this case, in S104 of FIG. 5, thecontrol module 141 acquires the input image from the imaging device 118so that the input image includes a medium having an amount which astraight line corresponding to the upper side of the medium and at leastone of a straight line corresponding to the right side of the medium ora straight line corresponding to the left side of the medium can bedetected. The detection module 143 detects only two straight linessubstantially perpendicular to each other in the step S203, and detectsonly the intersection of the two straight lines in the step S204. Inthis case, although the determination accuracy of whether or not themedium is an ID card is reduced, the medium conveying apparatus 100 canshorten the processing time of the card determination processing anddetermine whether or not the medium is an ID card at an early stage.

FIG. 8 is a flowchart illustrating an operation example of themulti-feed determination processing.

Referring to the flowchart illustrated in FIG. 8, an operation exampleof the multi-feed determination process in the medium conveyingapparatus 100 will be described below. The operation flow describedbelow is executed mainly by the processing circuit 140 in cooperationwith each element in the medium transport apparatus 100 in accordancewith a program previously stored in the storage device 130. Theflowchart illustrated in FIG. 8 is periodically executed, after thecontrol module 141 starts conveying the medium in the step S103 of FIG.5.

First, the control module 141 acquires an ultrasonic signal from theultrasonic sensor 114 (step S301).

Next, the control module 141 determines whether or not the type of themedium has been determined in the card determination process (stepS302).

When the type of the medium has not been determined, the controller 141stores the signal values of the acquired ultrasonic signals in thestorage device 130 (step S303), and ends a series of steps.

On the other hand, when the type of the medium has been determined, thecontrol module 141 determines whether or not it is determined that themedium is an ID card in the card determination process (step S304).

When it is determined that the medium is an ID card, the control module141 stops determining whether or not multi-feed has occurred (stepS305), and ends a series of steps.

On the other hand, when it is determined that the medium is not the IDcard, the control module 141 determines whether or not each signal valueof the ultrasonic signal acquired so far is less than the multi-feedthreshold value (step S306).

FIG. 9 is a schematic diagram for illustrating a characteristic of anultrasonic signal.

In a graph 900 in FIG. 9, a solid line 901 represents a characteristicof an ultrasonic signal when one sheet of paper is conveyed as a medium,and a dotted line 902 represents a characteristic of an ultrasonicsignal when multi-feed of paper is occurring. The horizontal axis ofgraph 900 indicates time, and the vertical axis indicates a signal valueof the ultrasound signal. Due to occurrence of multi-feed, a signalvalue of the ultrasonic signal in the dotted line 902 declines in asection 903. The multi-feed threshold value is set to a value between asignal value S1 of the ultrasonic signal when one sheet of paper isconveyed and a signal value S2 of the ultrasonic signal when multi-feedof paper is occurring. By determining whether or not a signal value ofthe ultrasonic signal is less than the multi-feed threshold value, thecontrol module 141 can determine whether or not the multi-feed of themedium is occurring.

On the other hand, the one-dot chain line 904 represents acharacteristics of the ultrasonic signal when only one plastic ID cardis conveyed. When an ID card is conveyed, since a signal value of theultrasonic signal becomes less than the multi-feed threshold value, thecontrol module 141 erroneously determines that multi-feed of the mediumhas occurred. In particular, a signal value of the ultrasonic signalwhen the multi-feed of thin paper is occurring is close to a signalvalue of the ultrasonic signal when an ID card is conveyed, and it isdifficult to set the multi-feed threshold value to a value between thetwo signal values. However, when it is determined that the medium is anID card, the control module 141 stops detecting the multi-feed, and thuscan prevent the detection error of the multi-feed of the medium.

The control module 141 determines that the multi-feed has occurred whenany of signal values of the ultrasonic signal acquired so far is lessthan the multi-feed threshold value (step S307), and ends series ofsteps. In this case, the control module 141 determines that multi-feedof the medium has occurred in the step S106 of FIG. 5, and controls theconveying roller so as to stop conveying the medium in the step S107. Onthe other hand, the control module 141 determines that the multi-feedhas not occurred when all of the signal values of the ultrasonic signalacquired so far is equal to or more than the multi-feed threshold value(step S308), and ends series of steps. Thus, the control module 141determines whether or not multi-feed of the medium has occurred bycomparing the ultrasonic signal with the multi-feed threshold value.

Further, the control module 141 stops determining whether or notmulti-feed has occurred, according to whether or not the conveyed mediumis an ID card. That is, the control module 141 controls the conveyingroller based on the result of the determination by the determinationmodule 144.

The control module 141 may control the conveying roller so that theconveying speed of the medium (the motor rotation speed of the drivingdevice 121) is different according to the result of the determination bythe determination module 144. In this case, the control module 141controls the conveying speed in the case where the conveyed medium is anID card to be lower (decelerate) than the conveying speed in the casewhere the conveyed medium is not an ID card, so that the slipperyplastic ID card is conveyed more stably.

Further, either the control of the conveying roller based on the resultof the determination by the determination module 144 or the output ofthe result of the determination may be omitted.

As described in detail above, the medium conveying apparatus 100 detectsthe degree of roundness at the corner of the medium based on the edgepixels in the input image, and determines whether or not the medium isan ID card based on the detected degree of roundness. As a result, themedium conveying apparatus 100 can more suitably determine whether ornot the conveyed medium is an ID card.

In particular, since the medium conveying apparatus 100 detects thedegree of roundness based on the distances between the intersection ofthe straight line corresponding to the outer shape of the medium and thepredetermined edge pixels, even when the medium is conveyed inclinedmanner, the medium conveying apparatus 100 can properly detect thedegree of roundness. Therefore, even when the medium is conveyed in aninclined manner, the medium conveying apparatus 100 can properlydetermine whether or not the medium is an ID card. Further, the mediumconveying device 100 can efficiently detect the degree of roundness witha low calculation amount by detecting the degree of roundness based onthe distances between the intersection of the straight linecorresponding to the outer shape of the medium and the predeterminededge pixels, thereby the medium conveying device 100 can reduce theprocessing load of the card determination process.

Further, since the user does not need to set the multi-feed detectionfunction to OFF in order to prevent the occurrence of the multi-feed ofthe medium from being erroneously detected when the medium conveyingapparatus 100 conveys a card, the medium conveying apparatus 100 canimprove the convenience of the user.

Further, since the medium conveying device 100 can determine whether ornot the medium is an ID card without using a special sensor such as athickness sensor, it is possible to suppress an increase in theapparatus cost.

FIG. 10 is a flowchart illustrating an operation example of carddetermination processing according to another embodiment.

The card determination processing shown in FIG. 10 is executed insteadof the card determination processing shown in FIG. 6. Since theprocessing of steps S401 to S402, S406 to S408 in FIG. 10 is similar tothe processing of steps S201 to S202, S207 to S209 in FIG. 6, detaileddescriptions thereof will be omitted, and only the processing of stepsS403 to S405 will be described below.

The detection module 143 extracts, for each of the upper edge pixels,the lower edge pixels, the left edge pixels, and the right edge pixelsextracted by the extraction module 142, a pair of two edge pixelsadjacent to each other among each edge pixel. The detection module 143calculates, for extracted pair, the inclination of the straight linepassing through the two edge pixels (step S403). That is, the detectionmodule 143 calculates the absolute value of the ratio of the changeamount of the position in the direction (vertical or horizontaldirection) perpendicular to a predetermined direction (horizontal orvertical direction) of the two edge pixels with respect to the changeamount of the position in the predetermined direction.

Next, the detection module 143 extracts two pairs adjacent to each otheramong the extracted pairs of the two edge pixels. The detection module143 calculates, for each of the extracted pairs, a degree of change ofthe inclination, which is a degree of change of each calculatedinclination (step S404). The detection module 143 calculates theabsolute value of the ratio of the change amount of the inclination withrespect to the change amount at the position in a predetermineddirection (horizontal or vertical direction) of the two edge pixels(used when calculating the inclination) as the degree of change of theinclination. The degree of change of the inclination calculated in thisway corresponds to a value (second order derivative value) acquired bysecond order derivative of the position in the predetermined directionof each edge pixel with respect to a direction perpendicular to thepredetermined direction. The detection module 143 may calculate theabsolute value of the change amount of the inclination as the degree ofchange of the inclination.

FIGS. 11A, 11B, 11C, and 11D are schematic diagrams for illustrating thedegree of change of the inclination of each edge pixel.

An image 1100 in FIG. 11A is an input image acquired by imaging an IDcard C conveyed straight. In the input image 1100, the upper edge pixelsare indicated by a solid line, the other edge pixels are indicated by adotted line. A graph 1101 in FIG. 11A is a graph showing the degree ofchange of the inclination of each upper edge pixel extracted from theinput image 1100. The horizontal axis of graph 1101 indicates theposition in the horizontal direction in the input image, and thevertical axis of graph 1101 indicates the degree of change of theinclination of each upper edge pixel at each position. As shown in theinput image 1100, in an ID card having a rounded corner, the inclinationof the upper edge pixel group changes nonlinearly with respect to thechange of the position in the horizontal direction in the portioncorresponding to the roundness. Therefore, as shown in the graph 1101,the ratio of the change amount of the inclination to the change amountof the position in the horizontal direction has a certain magnitude inthe portion corresponding to the roundness. On the other hand, althoughnot shown, in the paper in which the angle is right angle, theinclination of the upper edge pixel group does not change with respectto the change of the position in the horizontal direction.

The image 1110 in FIG. 11B is an input image acquired by imaging an IDcard C conveyed straight. In the input image 1110, the left edge pixelsare indicated by a solid line, the other edge pixels are indicated by adotted line. A graph 1111 in FIG. 11B is a graph showing the degree ofchange of the inclination of each left edge pixel extracted from theinput image 1110. The vertical axis of graph 1111 indicates the positionin the vertical direction in the input image, and the horizontal axis ofgraph 1111 indicates the degree of change of the inclination of eachleft edge pixel at each position. As shown in the input image 1110, inthe ID card having a rounded corner, the inclination of the left edgepixel group changes nonlinearly with respect to the change of theposition in the vertical direction in the portion corresponding to theroundness. Therefore, as shown in graph 1111, the ratio of the changeamount of the inclination to the change amount of the position in thevertical direction has a certain magnitude in the portion correspondingto the roundness. On the other hand, although not shown, in the paper inwhich the angle is right angle, the inclination of the left edge pixelgroup does not change with respect to the change of the position in thevertical direction.

An image 1120 in FIG. 11C shows an input image acquired by imaging an IDcard C conveyed in an inclined manner. In the input image 1120, theupper edge pixels are indicated by a solid line, the other edge pixelsare indicated by a dotted line. The graph 1121 of FIG. 11C is a graphshowing the degree of change of the inclination of each upper edge pixelextracted from the input image 1120. The horizontal axis of graph 1121indicates the position in the horizontal direction in the input image,and the vertical axis of graph 1121 indicates the degree of change ofthe inclination of each upper edge pixel at each position. As shown inthe input image 1120, as in the case where the ID card C is conveyedstraight, in the ID card having the roundness at the corner, theinclination of the upper edge pixel group changes nonlinearly withrespect to the change of the position in the horizontal direction in theportion corresponding to the roundness. Therefore, as shown in the graph1121, the ratio of the change amount of the inclination to the changeamount of the position in the horizontal direction has a certainmagnitude, in the portion corresponding to the roundness. On the otherhand, although not shown, in the paper in which the angle is rightangle, the inclination of the upper edge pixel group does not changewith respect to the change of the position in the horizontal directionexcept at one point corresponding to the angle.

The image 1130 of FIG. 11D shows an input image acquired by imaging anID card C conveyed in an inclined manner. In the input image 1130, theleft edge pixels are indicated by a solid line, the other edge pixelsare indicated by a dotted line. The graph 1131 of FIG. 11D is a graphshowing the degree of change of the inclination of each left edge pixelextracted from the input image 1130. The vertical axis of graph 1131indicates the position in the vertical direction in the input image, andthe horizontal axis of graph 1131 indicates the degree of change of theinclination of each left edge pixel at each position. As shown in theinput image 1130, as in the case where the ID card C is conveyedstraight, in the ID card having the roundness at the corner, theinclination of the left edge pixel group changes nonlinearly withrespect to the change of the position in the vertical direction in theportion corresponding to the roundness. Therefore, as shown in graph1131, the ratio of the change amount of the inclination to the changeamount of the position in the vertical direction has a certain magnitudein the portion corresponding to the roundness. On the other hand,although not shown, in the paper in which the angle is right angle, theinclination of the left edge pixel group does not change with respect tothe change of the position in the vertical direction except at one pointcorresponding to the angle.

Therefore, the detection module 143 can accurately detect the degree ofroundness at the corner of the medium included in the input image basedon the degree of change of the inclination of each edge pixel.

Next, the detection module 143 detects, for each of the upper edgepixels, the lower edge pixels, the left edge pixels, and the right edgepixels, the degree of roundness at the corner of the medium in the inputimage based on the degree of change of the inclination of each edgepixel (step S405). For example, the detection module 143 extracts, foreach of the upper edge pixels, the lower edge pixels, the left edgepixels, and the right edge pixels, the edge pixels from the firstpredetermined order to the second predetermined order in the order inwhich the degree of change of the inclination is large. The firstpredetermined order is the order of the second or subsequent order. Bysetting the first predetermined order to the second or subsequent order,the influence of the degree of change of the inclination calculated fromone point corresponding to the corner of the sheet is removed. Thesecond predetermined order is the order of the first predetermined orderor subsequent order. The detection module 143 calculates, for each ofthe upper edge pixel, the lower edge pixel, the left edge pixel, and theright edge pixel, a statistical value (average value, median value, modevalue, maximum value or minimum value, etc.) of the degree of change ofthe inclination of each extracted edge pixel. The detection module 143may calculate the number of edge pixels whose degree of change of theinclination is larger than a predetermined threshold value, as astatistical value of the degree of change of the inclination of eachedge pixel.

The degree of roundness is a value indicating a degree close to theideal value of the roundness at the corner of the ID card, and thedetection module 143 calculates the degree of roundness so that thedegree of roundness becomes higher as the calculated statistical valueis close to the ideal value of the ID card. For example, the detectionmodule 143 calculates the inverse of the difference between thecalculated statistical value and the ideal value, or a value acquired bysubtracting the difference from the upper limit value, as the degree ofroundness.

The ideal value in the ID card is set to the value of the statisticalvalue calculated for the ID card specified in ISO/IEC7810 by priorexperiments. The detection module 143 may set the degree of roundness to1 when the difference between the statistical value and the ideal valueis equal to or less than the predetermined threshold value, and may setthe degree of roundness to 0 when the difference between the statisticalvalue and the ideal value is larger than the predetermined thresholdvalue. The detection module 143 may set the degree of roundness to 1when the statistical value is equal to or larger than the ideal value,and may set the degree of roundness to 0 when the statistical value isless than the ideal value.

Instead of calculating the degree of roundness for all types of edgepixels of the upper edge pixel, the lower edge pixel, the left edgepixel and the right edge pixel, the detection module 143 may calculatethe degree of roundness only for at least one type of edge pixels. Inthat case, in the step S104 of FIG. 5, the control module 141 acquiresan input image from the imaging device 118 so that the input imageincludes a medium having an amount which at least the upper edge pixelscan be detected. The detection module 143 detects only the upper edgepixel in the step S402, calculates only the degree of change of theinclination related to the upper edge pixel in the step S403 and S404,and detects the degree of roundness only for the upper edge pixel in thestep S405. In this case, although the determination accuracy of whetheror not the medium is the ID card is reduced, the medium conveyingapparatus 100 can shorten the processing time of the card determinationprocessing and determine whether or not the medium is the ID card at anearly stage.

As described in detail above, the medium conveying device 100 can moresuitably determine whether or not the conveyed medium is an ID card evenwhen the medium conveying device 100 detects the degree of roundness atthe angle of the medium based on the degree of change of the inclinationof each edge pixel.

In particular, since the medium conveying device 100 detects the degreeof roundness based on the degree of change of the inclination of eachedge pixel, even when the medium is conveyed inclined manner, the mediumconveying device 100 can suitably detect the degree of roundness andsuitably determine whether or not the medium is an ID card. Further, themedium conveying device 100 can efficiently detect the degree ofroundness with a low calculation amount by detecting the degree ofroundness based on the degree of change of the inclination of each edgepixel, thereby the medium conveying device 100 can reduce the processingload of the card determination process.

FIG. 12 is a flowchart illustrating an operation example of the carddetermination process according to still another embodiment.

The card determination processing shown in FIG. 12 is executed insteadof the card determination processing shown in FIG. 6. Since theprocessing of steps S501 to S502, S507 to S509 in FIG. 12 is similar tothe processing of steps S201 to S202, S207 to S209 in FIG. 6, detaileddescriptions thereof will be omitted, and only the processing of stepsS503 to S506 will be described below.

The detector 143 detects, for each line in the input image, the mediumwidths based on a distance between the edge pixels included in each line(step S503). The detection module 143 specifies the edge pixels locatedat the leftmost end side and the edge pixels located at the rightmostend side among the edge pixels included in each line, and detects thedistance (the number of pixels) between the specified edge pixels as themedium width.

Next, the detection module 143 extracts a pair of two lines adjacent toeach other among each line in the input image. The detection module 143calculates, for each extracted pair, a first degree of change of themedium width which is a degree of change of the medium widths of the twolines (step S504). The detection module 143 calculates the absolutevalue of ratio of the change amount of the medium width in each linewith respect to the distance between the two lines as the first degreeof change of the medium width. The detection module 143 may calculatethe absolute value of the change amount of the medium width in each lineas the first degree of change of the medium width.

Next, the detection module 143 extracts two pairs adjacent to each otheramong the extracted pairs of the two lines. The detection module 143calculates, for each of the extracted pairs, a second degree of changeof the medium width which is a degree of change of the calculated eachfirst degree of change of the medium width (step S505). The detectionmodule 143 calculates the absolute value of ratio of the change amountof the first degree of change of the medium width in each line withrespect to the distance between the two lines (used when calculating thefirst degree of change of the medium width) as the second degree ofchange of the medium width. The detection module 143 may calculate theabsolute value of the change amount of the first degree of change of themedium width in each line as the second degree of change of the mediumwidth. The second degree of change of the medium width calculated inthis way corresponds to the second order derivative value of the mediumwidth in each line with respect to the direction perpendicular to eachline.

FIGS. 13A and 13B are schematic diagrams for illustrating the seconddegree of change of the medium width.

The image 1300 of FIG. 13A shows an input image acquired by imaging anID card C conveyed straight. Graph 1301 of FIG. 13A is a graph showingmedia widths detected from each line of input image 1300. The verticalaxis of graph 1301 indicates the line position in the vertical directionin the input image, and the horizontal axis of graph 1301 indicates themedia width at each line position. The graph 1302 of FIG. 13A is a graphshowing the first width variation in two adjacent lines. The verticalaxis of graph 1302 indicates the line position in the vertical directionin the input image, and the horizontal axis of graph 1302 indicates thesecond degree of change of the medium width at each line position. Thegraph 1303 of FIG. 13A is a graph showing the second degree of change ofthe medium width in two adjacent lines. The vertical axis of graph 1303shows the line position in the vertical direction in the input image,and the horizontal axis of graph 1303 shows the second degree of changeof the medium width at each line position.

As shown in graph 1302, in the ID card having roundness at the corner,the second degree of change of the medium width changes nonlinearly withrespect to changes of the position in the vertical direction, in theportion corresponding to the roundness. Therefore, as shown in the graph1303, the second degree of change of the medium width has a certainmagnitude in the portion corresponding to the roundness. On the otherhand, although not shown, in the paper in which the angle is rightangle, the medium width does not change, and the first degree of changeof the medium width and the second degree of change of the medium widthin all the lines become substantially 0.

An image 1310 in FIG. 13B shows an input image acquired by imaging an IDcard C conveyed in an inclined manner. Graph 1311 of FIG. 13B is a graphshowing the media width detected from each line of input image 1310. Thevertical axis of graph 1311 indicates the line position in the verticaldirection in the input image, and the horizontal axis of graph 1311indicates the media width at each line position. The graph 1312 of FIG.13B is a graph showing the first degree of change of the medium width intwo adjacent lines. The vertical axis of graph 1312 indicates the lineposition in the vertical direction in the input image, and thehorizontal axis of graph 1312 indicates the first degree of change ofthe medium width at each line position. The graph 1313 of FIG. 13B is agraph showing the second degree of change of the medium width in twoadjacent lines. The vertical axis of graph 1313 shows the line positionin the vertical direction in the input image, and the horizontal axis ofgraph 1313 shows the second degree of change of the medium width at eachline position.

As shown in graph 1312, in the ID card having roundness at the corner,the first degree of change of the medium width changes nonlinearly withrespect to changes of the position in the vertical direction, in theportion corresponding to the roundness. Therefore, as shown in the graph1313, the second degree of change of the medium width has a certainmagnitude in the portion corresponding to the roundness. On the otherhand, although not shown, in the paper in which the angle is rightangle, since the medium width varies linearly except at the four pointscorresponding to the corners, the first degree of change of the mediumwidth in each line becomes a constant value and the second degree ofchange of the medium width becomes approximately 0.

Therefore, the detection module 143 can accurately detect the degree ofroundness at the corner of the medium included in the input image basedon the second degree of change of the medium width in each line.

Next, the detection module 143 detects the degree of roundness at thecorners of the medium in the input image based on the second degree ofchange of the medium width in each line (step S506). For example, thedetection module 143 extracts lines whose order is from firstpredetermined order to second predetermined order in descending order ofthe second degree of change of the medium width. The first predeterminedorder is the order of the fifth or subsequent order. By setting thefirst predetermined order to the fifth or subsequent order, theinfluence of the second degree of change of the medium width calculatedfrom the four points corresponding to the corners of the sheet isremoved. The second predetermined order is the order of the firstpredetermined order or subsequent order. The detection module 143calculates a statistical value (average value, median value, mode value,maximum value or minimum value, etc.) of the second degree of change ofthe medium width in each extracted line. The detection module 143 maycalculate the number of lines having the second degree of change of themedium width larger than the predetermined threshold value as thestatistical value of the second degree of change of the medium width ineach line.

The degree of roundness is a value indicating a degree close to theideal value of the roundness at the corner of the ID card, and thedetection module 143 calculates the degree of roundness so that thedegree of roundness becomes higher as the calculated statistical valueis close to the ideal value of the ID card. For example, the detectionmodule 143 calculates the inverse of the difference between thecalculated statistical value and the ideal value, or a value acquired bysubtracting the difference from the upper limit value, as the degree ofroundness.

The ideal value in the ID card is set to the value of the statisticalvalue calculated for the ID card specified in ISO/IEC7810 by priorexperiments. The detection module 143 may set the degree of roundness to1 when the difference between the statistical value and the ideal valueis equal to or less than the predetermined threshold value, and may setthe degree of roundness to 0 when the difference between the statisticalvalue and the ideal value is larger than the predetermined thresholdvalue. The detection module 143 may set the degree of roundness to 1when the statistical value is equal to or larger than the ideal value,and may set the degree of roundness to 0 when the statistical value isless than the ideal value.

Instead of calculating the degree of roundness for all the lines, thedetection module 143 may calculate the degree of roundness only for theline including at least one corner region of the medium. In this case,the controller 141 acquires an input image from the imaging device 118so that the input image includes at least one rounded corner of themedium in S104 of FIG. 5. The detection module 143 detects the mediumwidth only in the lines including at least one corner area in step S503,and calculates the second degree of change of the medium width only forthe detected medium width in steps S504 and S505. Further, the detectionmodule 143 detects the degree of roundness only for the respective linesfor which the second degree of change of the medium width is calculatedin the step S506. In this case, although the determination accuracy ofwhether or not the medium is an ID card is reduced, the medium conveyingapparatus 100 can shorten the processing time of the card determinationprocessing and determine whether or not the medium is an ID card at anearly stage.

As described in detail above, the medium conveying apparatus 100 canmore suitably determine whether or not the conveyed medium is an ID cardeven when the medium conveying apparatus 100 detects the degree ofroundness at the corner of the medium based on the second degree ofchange of the medium width in each line.

In particular, since the medium conveying apparatus 100 detects thedegree of roundness based on the second degree of change of the mediumwidth in each line, even when the medium is conveyed in an inclinedmanner, the medium conveying apparatus 100 can suitably detect thedegree of roundness, and suitably determine whether or not the medium isan ID card. Further, the medium conveying device 100 can efficientlydetect the degree of roundness with a low calculation amount bydetecting the degree of roundness based on the second degree of changeof the medium width in each line, thereby the medium conveying device100 can reduce the processing load of the card determination process.

FIG. 14 is a flowchart illustrating an example of a part of an operationof the multi-feed determination process according to another embodiment.

The multi-feed determination processing shown in FIG. 14 is executedinstead of the multi-feed determination processing shown in FIG. 8.Since the processing of steps S601 to S604, S608 to S610 in FIG. 14 issimilar to the processing of steps S301 to S304, S306 to S308 in FIG. 8,detailed descriptions thereof will be omitted, and only the processingof steps S605 to S607 will be described below.

In the step S604, when it is determined that the medium is an ID card,the control module 141 determines whether or not each signal value ofthe ultrasonic signal acquired so far is less than the second multi-feedthreshold value (step S605). The second multi-feed threshold value isset to a value between the signal value S3 of the ultrasonic signal whenan ID card shown in FIG. 9 is conveyed and the signal value S2 of theultrasonic signal when the multi-feed of the paper is occurring so thatit can be determined whether an ID card is conveyed or the multi-feed ofthe paper is occurring. To prevent the occurrence of detection error ofmulti-feed, it is preferable that the multi-feed threshold value usedwhen an ID card is not conveyed is set to a value sufficiently largerthan the signal value S2 of the ultrasonic signal when the multi-feed ofthe paper is occurring. Therefore, the multi-feed threshold value is setto a value larger than the signal value S3 of the ultrasonic signal whenthe ID card is conveyed. That is, the second multi-feed threshold valuewhen the medium is an ID card is set to a value less than the multi-feedthreshold value when the medium is not an ID card.

When any of the signal values of the ultrasonic signal acquired so faris less than the second multi-feed threshold value, the control module141 determines that the multi-feed has occurred (step S606), and endsseries of steps. In this case, the control module 141 determines thatmulti-feed of the medium has occurred in the step S106 of FIG. 5, andcontrols the conveying roller to stop conveying the medium in the stepS107. On the other hand, when all of the signal values of the ultrasonicsignal acquired so far is equal to or larger than the second multi-feedthreshold value, the control module 141 determines that the multi-feedhas not occurred (step S607), and ends series of steps. In this manner,the control module 141 changes the multi-feed threshold value fordetermining whether or not multi-feed has occurred, according to whetheror not the conveyed medium is an ID card.

As described in detail above, the medium conveying apparatus 100 cansuitably determine whether or not the medium to be conveyed is an IDcard, and suitably control the conveying roller even when the mediumconveying apparatus 100 changes the multi-feed threshold value accordingto whether or not the medium is an ID card.

The medium conveying apparatus 100 may execute not only one of the carddetermination processes shown in FIGS. 6, 10, and 12, but also acombination of two or more of the card determination processes. In thiscase, the medium conveying apparatus 100 calculates the sum, product, orweighted sum of the degrees of roundness detected in each carddetermination process as a new degree of roundness, and determineswhether or not the conveyed medium is an ID card based on whether or notthe calculated new degree of roundness is larger than a roundnessthreshold value. As a result, the medium transport apparatus 100 canmore accurately determine whether or not the medium is an ID card.

The medium conveying apparatus 100 may further determine whether or notthe conveyed medium is an ID card based on the size (width and length)of the medium. In that case, when the degree of roundness is larger thanthe roundness threshold value, the determination module 144 calculatesthe size (width and length) of the medium from the distance between theintersections detected in the same manner as the step S204 of FIG. 6.The determination module 144 calculates the degree of similarity betweenthe calculated size and the size of the ID card (85.60 mm×53.98 mm)defined by ISO/IEC7810. The degree of similarity is, for example, theinverse of the difference between the sizes. The determination module144 determines that the conveyed medium is an ID card when the degree ofsimilarity is equal to or larger than a preset size threshold value, anddetermines that the conveyed medium is not an ID card when thesimilarity is less than the size threshold value. As a result, themedium transport apparatus 100 can more accurately determine whether ornot the medium is an ID card.

FIG. 15 is a diagram illustrating a schematic configuration of aprocessing circuit 240 in a medium conveying apparatus according tostill another embodiment. The processing circuit 240 is used in place ofthe processing circuit 140 of the medium conveying apparatus 100 andincludes a control circuit 241, an extraction circuit 242, a detectioncircuit 243, a determination circuit 244, etc.

Note that each unit included in the processing circuit may be configuredby an independent integrated circuit, a microprocessor, firmware, etc.In addition, a part of the processing circuit may be composed of acircuit, and a part of the processing circuit may be composed of afunctional module implemented by software running on a processor.

The control circuit 241 is an example of a control module, and has afunctions similar to the control module 141. The control circuit 241receives an operation signal from the operating device 105, a firstmedium signal from the first medium sensor 111, and a second mediumsignal from the second medium sensor 117, and drives the driving device121 to control the conveying roller in response to each received signal.In addition, the control circuit 241 receives an input image from theimaging device 118 and stores it in the storage device 130, and receivesa read image and outputs it to an information processing apparatus (notshown) via the interface device 122. Further, the control circuit 241receives an ultrasonic signal from the ultrasonic sensor 114, determineswhether or not multi-feed of the medium has occurred based on theultrasonic signal, and controls the conveying roller to stop conveyingthe medium when it is determined that multi-feed of the medium hasoccurred. In addition, the control circuit 241 reads out the result ofthe determination as to whether or not the conveyed medium is an ID cardfrom the storage device 130, controls the conveying roller based on theresult of the determination, or outputs the result of the determinationto an information processing apparatus (not shown) via the interfacedevice 122.

The extraction circuit 242 is an example of an extraction module, andhas a functions similar to the extraction module 142. The extractioncircuit 242 reads an input image from the storage device 130, extractsedge pixels from the input image, and stores the extraction result inthe storage device 130.

The detection circuit 243 is an example of a detection module, and has afunctions similar to the detection module 143. The detection circuit 243reads out the input image and the extraction result of the edge pixelfrom the storage device 130, detects the degree of roundness at thecorner of the medium in the input image based on the edge pixel, andstores the detection result in the storage device 130.

The determination circuit 244 is an example of a determination modulehas a functions similar to the determination module 144. Thedetermination circuit 244 reads out the detection result of the degreeof roundness from the storage device 130, determines whether or not theconveyed medium is an ID card based on the degree of roundness, andstores the result of the determination in the storage device 130.

As described in detail above, even when the processing circuit 240 isused, the medium conveying apparatus can more suitably determine whetheror not the conveyed medium is an ID card.

According to this embodiment, the medium conveying apparatus, themethod, and the computer-readable, non-transitory medium storing thecontrol program, can more suitably determine whether or not a conveyedmedium is an ID card.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiment(s) of the presentinventions have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A medium conveying apparatus comprising: aconveying roller to convey a medium; an imaging device to generate aninput image acquired by imaging the conveyed medium; and a processor toextract edge pixels from the input image, detect two straight linessubstantially perpendicular to each other from the edge pixels,determine whether the conveyed medium is a predetermined card based on adistance between an intersection of the two straight lines andpredetermined edge pixels at a corner of the medium in the input image,and control the conveying roller based on a result of the determinationor outputs the result of the determination.
 2. The medium conveyingapparatus according to claim 1, further comprising an ultrasonic sensorincluding an ultrasonic transmitter for transmitting an ultrasonic waveand an ultrasonic receiver facing the ultrasonic transmitter forreceiving the ultrasonic wave and generating an ultrasonic signalcorresponding to the received ultrasonic wave, wherein the processorfurther: determines whether multi-feed of the medium has occurred bycomparing the ultrasonic signal with a threshold value, controls theconveying roller to stop conveying the medium when the processordetermines that multi-feed of the medium has occurred, and changes thethreshold value or stops determining whether multi-feed has occurred,according to whether the conveyed medium is the predetermined card. 3.The medium conveying apparatus according to claim 1, wherein theprocessor further: calculates, for each pair of two edge pixels adjacentto each other among the edge pixels, an inclination of a straight linepassing through the two edge pixels, calculates a degree of change ofthe inclination, for two pairs of edge pixels adjacent to each other,and determines whether the conveyed medium is the predetermined cardbased on the degree of change of the inclination.
 4. The mediumconveying apparatus according to claim 1, wherein the processor further:detects, for each line in the input image, a medium width based on adistance between edge pixels included in each line, calculates, for eachpair of two lines adjacent to each other among the lines, a first degreewhich is a degree of change of the medium widths in the two lines,calculates a second degree which is a degree of change of the firstdegrees, for two pairs of the two lines adjacent to each other, anddetermines whether the conveyed medium is the predetermined card basedon the second degree.
 5. The medium conveying apparatus according toclaim 1, wherein the predetermined card is an ID card or a plastic card.6. A medium conveying apparatus comprising: a conveying roller to conveya medium; an imaging device to generate an input image acquired byimaging the conveyed medium; and a processor to extract edge pixels fromthe input image, detect two straight lines substantially perpendicularto each other from the edge pixels, detect a shape of a corner of themedium in the input image based on a distance between an intersection ofthe two straight lines and predetermined edge pixels, determine whetherthe conveyed medium is a predetermined card based on the shape of thecorner, and control the conveying roller based on a result of thedetermination or outputs the result of the determination.
 7. The mediumconveying apparatus according to claim 6, further comprising anultrasonic sensor including an ultrasonic transmitter for transmittingan ultrasonic wave and an ultrasonic receiver facing the ultrasonictransmitter for receiving the ultrasonic wave and generating anultrasonic signal corresponding to the received ultrasonic wave, whereinthe processor further: determines whether multi-feed of the medium hasoccurred by comparing the ultrasonic signal with a threshold value,controls the conveying roller to stop conveying the medium when theprocessor determines that multi-feed of the medium has occurred, andchanges the threshold value or stops determining whether multi-feed hasoccurred, according to whether the conveyed medium is the predeterminedcard.
 8. The medium conveying apparatus according to claim 6, whereinthe processor further: calculates, for each pair of two edge pixelsadjacent to each other among the edge pixels, an inclination of astraight line passing through the two edge pixels, calculates a degreeof change of the inclination, for two pairs of edge pixels adjacent toeach other, and detects the shape of the corner based on the degree ofchange of the inclination.
 9. The medium conveying apparatus accordingto claim 6, wherein the processor further: detects, for each line in theinput image, a medium width based on a distance between edge pixelsincluded in each line, calculates, for each pair of two lines adjacentto each other among the lines, a first degree which is a degree ofchange of the medium widths in the two lines, p1 calculates a seconddegree which is a degree of change of the first degrees, for two pairsof the two lines adjacent to each other, and detects the shape of thecorner based on the second degree.
 10. The medium conveying apparatusaccording to claim 6, wherein the predetermined card is an ID card or aplastic card.
 11. A medium conveying apparatus comprising: a conveyingroller to convey a medium; an imaging device to generate an input imageacquired by imaging the conveyed medium; and a processor to extract edgepixels from the input image, detect two straight lines substantiallyperpendicular to each other from the edge pixels, detect a state of acorner of the medium in the input image based on a distance between anintersection of the two straight lines and predetermined edge pixels,determine whether the conveyed medium is a predetermined card based onthe state of the corner, and control the conveying roller based on aresult of the determination or outputs the result of the determination.12. The medium conveying apparatus according to claim 11, furthercomprising an ultrasonic sensor including an ultrasonic transmitter fortransmitting an ultrasonic wave and an ultrasonic receiver facing theultrasonic transmitter for receiving the ultrasonic wave and generatingan ultrasonic signal corresponding to the received ultrasonic wave,wherein the processor further: determines whether multi-feed of themedium has occurred by comparing the ultrasonic signal with a thresholdvalue, controls the conveying roller to stop conveying the medium whenthe processor determines that multi-feed of the medium has occurred, andchanges the threshold value or stops determining whether multi-feed hasoccurred, according to whether the conveyed medium is the predeterminedcard.
 13. The medium conveying apparatus according to claim 11, whereinthe processor further: calculates, for each pair of two edge pixelsadjacent to each other among the edge pixels, an inclination of astraight line passing through the two edge pixels, calculates a degreeof change of the inclination, for two pairs of edge pixels adjacent toeach other, and detects the state of the corner based on the degree ofchange of the inclination.
 14. The medium conveying apparatus accordingto claim 11, wherein the processor further: detects, for each line inthe input image, a medium width based on a distance between edge pixelsincluded in each line, calculates, for each pair of two lines adjacentto each other among the lines, a first degree which is a degree ofchange of the medium widths in the two lines, calculates a second degreewhich is a degree of change of the first degrees, for two pairs of thetwo lines adjacent to each other, and detects the state of the cornerbased on the second degree.
 15. The medium conveying apparatus accordingto claim 11, wherein the predetermined card is an ID card or a plasticcard.